Ultraviolet-visible absorption bands

Tab. 7.5 Ultraviolet-visible absorption bands and electron transitions for the iron oxides (data for magnetite, A/ustite and akaganeite from Strens Wood, 1979 A/ith permission bernalite from McCammon et al.,1995 remainder from Sherman Waite, 1985 A/ith permission)...

Ultraviolet-visible absorption bands (in deionized water) appear at 440 (w), 322 (s), and 234 (s) nm, in close agreement with published data for the chloride. ... 

Since we had proposed a similar experiment with irradiation in the ultraviolet-visible absorption band of the uranyl ion (15), we tried to reproduce these results, but without success (16). Collisions in the liquid phase occur so rapidly (about 10l2 s x) that vibrational excitation of the uranyl ions would be dissipated long before any significant fraction of excited uranyl ions could reach the interface and therefore change the distribution between the two phases. Rapid loss of vibrational excitation in relation to other processes is a generic problem for infrared laser effects in any system of condensed phases. However, differences between experimental setups may account for the differences in results,... 

Ultraviolet-visible absorption has traditionally been the basis of detection in flash photolysis experiments. It offers a number of advantages in sensitivity and efficiency and has certainly delivered much vital information about the reactivity of transient species. On the other hand, the ultraviolet-visible absorption bands characteristic of any but the simplest molecules tend to be broad and relatively uninformative as regards identity and structure, and so we may run into problems not only with the overlap of absorptions due to different... 

UV (characteristic Ultraviolet visible absorption bands for polyene sequences with (9)... 

For the purpose of measuring the infrared spectra of unstable species such as free radicals in solution, it is advisable to place a small Fourier transform infrared (FT-IR) spectrometer into an inert-gas glovebox system, in which the concentrations of oxygen and water vapor can be kept at <0.1 ppm. In such an experimental setup, it is possible to generate unstable species in appropriate solvents and measure their infrared spectra in solution [6]. Preferably, a small ultraviolet/visible spectrometer should also be placed within the glove-box system. Then, it will be possible to determine the concentration of the unstable species under study by additionally measuring its ultraviolet/visible absorption spectrum, provided the molar absorption coefficient of the ultraviolet/visible absorption band of the unstable species is known. 

LSPR are collective charge oscillations that occur at the interface between conductors and dielectrics of nanometersized metallic structures causing unique properties such as strong ultraviolet/visible absorption bands that are not present in the planar metal or brilliant colors observed for the particles in solution. It is this fascination with the optical properties that lead to their many applications in the development of biosensors. 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

The solutions usually cannot be cleaned up by extraction techniques because hydralazine decomposes in alkaline solution. However, there are many reactions that give rise to near ultraviolet or visible absorption bands suitable for quantitation. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

Table I. Ultraviolet and Visible Absorption Bands of Cu(III) Complexes...

Many attempts have already been made to arrive at a quantitative interpretation of the position of the ultraviolet and visible absorption bands. 

Free-base corroles as well as their anionic and monoprotonated adducts are generally characterized by several strong ultraviolet (UV)-visible absorption bands. As is true for the porphyrins, the position and intensity of these bands presumably reflects the extended aromatic conjugation present within the molecule. Indeed the spectra of corroles resemble those of the porphyrins in that there is typically a strong Soret-like transition near 400 nm, as well as three weaker, long-wavelength Q-type transitions in the 500-600 nm spectral region. ... 

As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a function of size is most directly performed by ultraviolet-visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct band gap in the material. As the band gap shifts to higher energy, the blue-shift in the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV-Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in Section 11.3. 

Color centers can be produced in the alkali metal azide by ultraviolet light and ionizing radiation at low temperatures. The phenomenon has been of interest for some time since the defects produced are involved in the process of photochemical decomposition (cf. Chapter 7). In earlier studies [54a, b, c] purely speculative identifications of optical absorption bands with F, V, and aggregate F centers were made by analogy with the alkali halides. The most prominent visible absorption band in each case was attributed to the F center—a defect involving an electron trapped at an azide (N3) vacancy. In the case of NaNa, spin resonance [55] and recent point ion calculations [56] clearly point to the existence of a F center. However, in the case of KN3, spin-resonance studies [54a] point to the existence of molecular centers of type N2 (on low-temperature irradiation) and NJ (on room-temperature irradiation). Infrared absorptions [57] and Raman scattering [58] have been observed in the irradiated alkali azides, which can be correlated with modes associated with these defects. 

Like the porphyrins, phthalocyanines absorb in the near-ultraviolet and visible region, but the intensities of the absorptions are entirely different. It is the visible absorption bands that are more intense than the near-ultra-violet bands, not the other way round, as with porphyrins. The reasons for this are perturbations to the phthalocyanine ir-system caused by, (a) the nitrogen atoms in the meso-positions (they are more electronegative than carbon atoms so that they tend to attract -ir-electron density towards themselves) and, (b) the fused benzene rings on the pyrrole 3-positions, which extend the -iT-system (they increase the size of the electron "box"). 

The absorption characteristics have been used to estimate the number of nitroazidophenyl groups that are attached to protein derivatized with 4-azido-2-nitrofluorobenzene. The ultraviolet-visible absorption or the characteristic infrared band at approximately 2100 cm-, often broad or a doublet, can be used to follow photolyses. 

It is usually easy to apply the single-band method to ultraviolet-visible absorption spectra, as only one or two bands of the analyte are observed. In ultraviolet-visible absorption measurements, it is possible to use a solvent that has no absorption band within this spectral region. By contrast, it is not always so easy to apply the single-band method to infrared absorption spectra, which usually consist of a number of bands arising not only from the solute but also from the solvent. If a sample contains two or more dissolved substances as solutes, the observed spectrum becomes even more complex. Many bands may overlap other bands at least partially. In such a case, a key band for quantitative analysis should be selected with care and its intensity should be measured in an appropriate manner as described in Section 3.4.2. It is necessary to make the signal-to-noise ratio of... 

Determination of purity. The ultraviolet and visible absorption is often a fairly intensive property thus e values of high intensity bands may be of the order of 10 -10 . In infrared spectra e values rarely exceed 10 . It is therefore often easy to pick out a characteristic band of a substance present in small concentration in admixture with other materials. Thus small amounts of aromatic compounds can be detected in hexane or in cyclohexane. 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

The determination of an analyte s concentration based on its absorption of ultraviolet or visible radiation is one of the most frequently encountered quantitative analytical methods. One reason for its popularity is that many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, analytes that do not absorb UV/Vis radiation, or that absorb such radiation only weakly, frequently can be chemically coupled to a species that does. For example, nonabsorbing solutions of Pb + can be reacted with dithizone to form the red Pb-dithizonate complex. An additional advantage to UV/Vis absorption is that in most cases it is relatively easy to adjust experimental and instrumental conditions so that Beer s law is obeyed. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Cerous salts in general are colorless because Ce " has no absorption bands in the visible. Trivalent cerium, however, is one of the few lanthanide ions in which parity-allowed transitions between 4f and Sd configurations can take place and as a result Ce(III) compounds absorb in the ultraviolet region just outside the visible. 

Substance A has an absorption spectmm in one or more regions of the ultraviolet or visible spectral range. Irradiation of A at a wavelength corresponding to one of the absorption bands results in formation of substance B, which has a visible absorption spectmm different from A. Most commonly, substance A is uncolored or only slightly colored, whereas substance B is colored or appears darker than A. 

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